The present invention relates generally to the fabrication of semiconductor devices and more particularly to chip attachment for precision aligned macro (PAM) applications.
A xe2x80x9cchipxe2x80x9d is a semiconductor body in which an integrated circuit is formed or is to be formed; often, the word xe2x80x9cchipxe2x80x9d and the term xe2x80x9cintegrated circuitxe2x80x9d are used interchangeably. The fabrication of precision aligned macros (PAMs) involves joining a series of previously diced chips into a precision-aligned planer array on a common substrate, and wiring these diced chips together. PAMs can be used to provide a wide variety of devices with incompatible fabrication processes on a single chip. Existing PAM fabrication techniques suffer from a susceptibility, however, to electrostatic discharge damage and thermal damage.
One approach to making planer arrays of diced chips (or segments) is disclosed by H. Bernard Pogge et al. in U.S. Pat. No. 6,025,638. The approach uses a removable mandrel (alignment wafer) to align the diced chips, fills the gaps between chips with a dielectric material, planarizes the back end of the diced chips using chemical mechanical polishing, and bonds a carrier wafer to the planer back ends of the diced chips. Pogge et al. do not provide a process for reducing the risk of electrostatic discharge damage or thermal damage during interconnect wiring of the diced chips. Moreover, although the process taught by Pogge et al. can planarize diced chips with thickness variations due to process variability, chemical mechanical polishing can damage diced chips which are substantially thicker than surrounding diced chips by design.
Another approach to making planer multi-chip modules is disclosed by Yaw-Hwang Chen in U.S. Pat. No. 5,506,383. Chen forms a multi-chip module by placing chips into openings previously formed in a wafer substrate. Interconnect wiring has also been previously formed in the wafer substrate. The chips are bonded to the wafer with conductive solder then cooled from the back side through the openings in the wafer. The disclosed formation process suffers from several problems.
First, chip interconnections are only provided through spun-on gold bridges to solder pads on the wafer and chips. Such bridges are not compatible with high density wiring used for precision aligned macros. Second, the solder used to bond the chips to the wafer may be melted by typical wafer-level wiring, releasing the chips from their positions. Third, multiple through holes in the wafer for chip placement and cooling may cause the wafer to be too fragile for wafer-level wiring processes, such as spin apply resist. Fourth, although thermal dissipation and presumably electrostatic discharge protection can be provided through a heat sink bonded to the back faces of the chips, the disclosed technique is not compatible with chips of different thicknesses because the back faces of the chips will not be co-planer when the faces of the chips are coplaner for bridging between the chips and wafer.
Yet another approach to making planer arrays of dissimilar chips (or segments) is disclosed by John W. Sliwa, Jr. in U.S. Pat. No. 5,075,253. This approach uses a liquid surface-tension driven flotation and assembly mechanism. Sliwa suggests the use of an alignment plate to force the top surfaces of the segments to be co-planer. Sliwa does not disclose or suggest, however, reducing damage caused by electrostatic discharge or thermal dissipation. Nor does Sliwa provide a process for maintaining co-planarity of the tops of chips with different thicknesses.
The importance of overcoming the susceptibility to electrostatic discharge damage is evidenced by the extensive technological development directed to the subject. Steven Howard Voltman et al. teaches a technique, in U.S. Pat. No. 5,930,098, for electrostatic discharge suppression on a chip stack by connecting power plane to power plane and external connect to external connect with conductive busses on the chip edges. This technique is not compatible, however, with planer arrays.
The deficiencies of the conventional devices and fabrication processes show that a need still exists for an improved structure and process for multi-chip chip attach with reduced risk of electrostatic discharge damage and thermal damage. To overcome the shortcomings of the conventional devices and fabrication processes, a new device and fabrication process are provided. An object of the present invention is to provide a precision aligned macro with reduced susceptibility to electrostatic discharge and thermal damage. It is a further object of the present invention to provide a precision aligned macro with improved co-planarity of the faces of the different diced chips. It is yet another object of the present invention to provide a process for fabricating a precision aligned macro according to the previous objects which is compatible with high-density wiring using conventional wafer-level wiring processes.
To achieve these and other objects, and in view of its purposes, the present invention provides a structure and process for fabricating precision aligned macros (PAMs) with reduced risk of electrostatic discharge damage and thermal damage. The structure and process of the present invention further provide thermal dissipation. An electrical and thermal contact is provided through the back of the individual chips to a supporting silicon substrate.
The process of the present invention includes at least the following steps. A seed layer for electroplating is formed on a support substrate. A thermid layer is formed on the seed layer. Vias are formed in the thermid layer and metal contacts are formed in the vias. The front faces of two or more chips are bonded onto the top surface of an alignment substrate, in which the chips are aligned to the alignment substrate. The back faces of the chips are bonded to the metal contacts and thermid layer with heat and pressure. The alignment substrate is removed. The front faces of the chips are planarized. Interconnect wiring is formed over the chips and thermid layer.
The present invention provides considerable improvement over the prior art. An electrically and thermally conductive path is provided to dissipate electrostatic charge and heat build-up through the back faces of the chips. Also, the present invention provides a co-planer surface at the front faces of the chips and the top of the thermid layer, suitable for forming high-density interconnects using conventional photolithography processes. Photolithography is a process in which a light source illuminates a circuit pattern and projects the image through a lens assembly onto a semiconductor wafer or substrate. Ultimately, the circuit pattern is etched into the wafer.
It should be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the present invention.